Electronic displays can use transmissive or emissive materials to generate pictures or light. Emissive materials are usually phosphorescent or electroluminescent materials. Examples are inorganic electroluminescent materials such as applied in thin film and thick film electroluminescent displays (EL-displays, for example thin film TFEL displays as manufactured by Sharp, Planar, LiteArray or iFire/Westaim). Another group is organic electroluminescent materials (such as Organic Light Emitting Diode (OLED) material) deposited in layers comprising small molecule or polymer technology or phosphorescent OLED, where the electroluminescent materials are doped with a phosphorescent material. Yet another group of materials are phosphors, commonly used in the well-established cathode ray tubes (CRT) or plasma displays (PDP) and even in emerging technologies like laser diode projection displays where a laser beam is used to excite a phosphor imbedded in a projection screen.
Two basic types of displays exist: fixed format displays which comprise a matrix or array of “cells” or “pixels” each producing or controlling light over a small area, and displays without such a fixed format, e.g. a CRT display. For fixed format, there is a relationship between a pixel of an image to be displayed and a cell of the display. Usually this is a one-to-one relationship. Each cell may be addressed and driven separately. Emissive, fixed format especially direct view displays such as Light Emitting Diode (LED), Field-Emission (FED), Plasma, EL, OLED and Polymeric Light Emitting Diode (PLED) displays have been used in situations where conventional CRT displays are too bulky and/or heavy and provide an alternative to non-emissive displays such as Liquid Crystal displays (LCD). Fixed format means that the displays comprise an array of light emitting cells or pixel structures that are individually addressable, rather than using a scanning electron beam as in a CRT. Fixed format relates to pixelation of the display as well as to the fact that individual parts of the image signal are assigned to specific pixels in the display. Even in a colour CRT, the phosphor triads of the screen do not represent pixels; there is neither a requirement nor a mechanism provided, to ensure that the samples in the image in any way align with these. The term “fixed format” is not related to whether the display is extendable, e.g. via tiling, to larger arrays. Fixed format displays may include assemblies of pixel arrays, e.g. they may be tiled displays and may comprise modules made up of tiled arrays which are themselves tiled into super-modules. Thus “fixed format” does not relate to the fixed size of the array but to the fact that the display has a set of addressable pixels in an array or in groups of arrays. Making very large fixed format displays as single units manufactured on a single substrate is difficult. To solve this problem, several display units or “tiles” may be located adjacent to each other to form a larger display, i.e. multiple display element arrays are physically arranged side-by-side so that they can be viewed as a single image. Transferring image data by packetised data transmission to the various display devices makes segregation of the displayed image into tiles relatively easy.
When making colour displays, the colours are obtained through mixing light from primary colours such as, but not limited to, red (R), green (G) and blue (B). For fixed format emissive displays separate or stacked individual “primary” emitter layers generate these colours. If the primary emitter layers are applied next to each other and usually close to each other, then from a certain minimum distance onwards (compounding distance), an observer is not able to distinguish the primary emitters but sees only one resulting mixed colour. Most colour displays are bicolour or full colour, referring to respectively two primaries or at least three primary emitters per pixel.
In order to be able to generate as many colours as possible, including white, at least three primary emitters are required with the emitted wavelengths of each as close as possible to pure colours such as pure red, pure green and pure blue, for example. The theory of colour perception is well known, for example from the book “Display Interfaces”, R. L. Myers, Wiley, 2002. Primaries exist as mathematical constructs only, which lie outside the range of real-world colours. A more useful colour space and colour co-ordinate system has been standardised, e.g. the CIE chromaticity diagram. Typically in fixed format displays red, green and blue pixel elements are used, typically called RGB pixel elements. A CIE chromaticity diagram with the locations thereon of typical OLED and LED materials (respectively graphs 10 and 11) is shown in FIG. 1. The locations on this diagram are shown for a typical OLED display (graph 10): red, RO; green, GO and blue, BO as well as for an LED display (graph 11): red, RL; green, GL; blue, BL.
Finally, emitters for fixed format displays have a certain emissive spectrum. Each material has a different dominant wavelength as well. This determines unambiguously what colours can be generated with a pixel.
It is known that a plurality of LEDs, and a plurality of OLEDs, show a variation in their emissive spectrum (e.g. due to fluctuation in the production process), as can be seen on FIG. 1. As the human eye is very sensitive to colour differences, colour variations between the many pixels may become perceptible, creating a distracting artefact known as “fixed pattern noise” or dithering.
In addition, in the course of differential ageing, the individual sub-pixels may change luminous efficiency and/or colour differently. If the luminous efficiency and/or colours of the sub-pixels change during ageing, and all the sub-pixels do not age in substantially the same way, a colour and/or luminance difference may also become more perceptible over time.
U.S. 2003/0443088 describes a solution to the above problem. For a given display, the colour of each sub-pixel is characterised in the factory as part of the final test before shipping. The expressed colour for each pixel is set to the smallest colour gamut for the complete population of pixels. In other words, emitted colour from each pixel is limited to the smallest colour gamut which all of the pixels of in the display can achieve.
This approach assumes substantial uniformity of the colours shown by all of the pixels of the display device. However, it sacrifices the potential colour gamut possible with a given display.